As an insulating material for use in solid material-insulated cables, cross-linked polyethylene prepared by chemical cross-linking, i.e., using cross-linking agents, is ordinarily used.
In this chemical cross-linking, dicumyl peroxide (DCP) is used as a cross-linking agent because of its high cross-linking efficiency and suitable decomposition temperature. In chemically cross-linked polyethylene-insulated cables, in particular, those cables in which the thickness of the insulating layer is large, if voltage is applied to at high temperatures (e.g., 90.degree. C.), the breakdown voltage of the cables decreases even if cables are not used in the water.
In particular, in the case of cables with a metal layer as a water-shielding layer provided around an insulating layer, the breakdown voltage abruptly decreases when voltage is applied to at much higher temperatures.
The above-described problem of the decrease in breakdown voltage is encountered particularly when cross-linking is performed by a method using high pressure steam which has been generally employed. The so-called dry cross-linking method (a method of chemically cross-linking by the use of, for example, a heat transfer medium such as silicon oil, infrared radiation, hot inert gas, or a long die) can prevent the decrease in breakdown voltage of the cable with a thin insulation (less than 6 mm).
A cross-linking system for cross-linking polyethylene insulated cables using pressurized silicone oil as a medium for heating and cooling is described in U.S. Pat. No. 3,909,177.
Systems for heating cables with infrared radiation are described in U.S. Pat. Nos. 3,588,954 and 3,513,228.
Cross-linking systems using hot inert gas are described in U.S. Pat. No. 4,069,286 and IEEE Trans. Vol. PAS-94 No. 2 (1978), T74 394-3.
The above systems have been developed as a substitute for a cross-linking system using steam in order to prevent the penetration of water into an insulating layer.
However, when voltage is applied to at high temperatures, the problem of decrease in breakdown voltage cannot be overcome satisfactorily even by employing the dry cross-linking method.